Production and transmission of modulated waves



June 15 1926.

v. I... HARTLEY PRODUCTION AND TRANSMISSION OF MODULATED WAVES Filed Dec. -27. 1921 Patented a .1926; 1

I D ATES PATENT" OFFICE.

34m n v. 1.. HARTLEY, or nnsr ORANGE, NEW JERSEY, ASSIGNOB TO WESTERN ELIO- TBJC COMPANY, INCORPORATED, 0] NEW YORK, N. Y., A. OOB-IORA'JION' OII NEW YORK.

PRODUCTION AND TRANSMISSION OF MODOLLTED WAVES.

Application filed December 27, 1821. Serial Io. 585,018.

The present invention relates to the production of modulated waves by commutation of the signaling currents, and compr ses both means and method therefor.

The invention further relates to the transmission and reception of the modulated waves so produced, including method and means of purifying the waves for transmlssion and reception, controlling transmission of different waves or different portions of the same wave selectively, and accomplishing other effects such as will appear from the more detailed description to follow.

If alternating current is produced by com mutating a direct current, it is known that the amplitude ofthe alternating current varies according to the magnitude or strength of the direct current. From a varying direct current an alternating current of varying amplitude can therefore be produced. When the direct current is a signaling current, a signal modulated wave may be produced by commutation. The signaling current need not bea varying direct current, however, for if it be an alternating current, the resulting high frequency Wave produced by commutation of the signal current will begsubstantially a pure modulated wave as explained later. The commutator may, therefore, be used as a modulator in carrier current transmission, and this type of modulator possesses some distinct advantages over types of modulators heretofore used, such as the vacuum tube. These advantages will be pointed out later on in the detailed description.

To reconvert the alternating current wave into current of the signal frequency any suitable type of detector or rectifier may be used, but since a commutator acts as a rectifier of alternating current if run in synchronism with the current alternations, a

commutator can be used for detecting the.

received wave) Further, the same commutator used to produce an outgoing modu lated wave may be used to rectify the incoming waves. A channel may terminate at each end in a commutator which serves both to modulate and detect, these commutators being run in synchronism with each other in any suitable manner. For multiplex oporation different simultaneous message currents are commutated at different rates and.

filters are used to separate the current'cominverted voice frequencies ponents of the different frequencies employed by the res ective channels.

For a better on erstanding of the various features of the invention and its exact nature and objects, the following detailed description and the appended claims may be consulted, reference being bad to the accompanying drawings in. which Figs. 1 and 2- show curves illustrative of the manner of,

operation of a commutator in modulating and detecting; Fig, 3 is a diagrammatic circuit arrangement of a carriertelephone sys- 'tem'of one channel, preferably employing for purposes of secrecy; and Fig. 4 is a similar representation of a multiplex system according to the invention. 1

In Fig. 1, the curve 1 may be assumed to represent a portion of a varyin direct current signal. .If a current of this character is applied to a rapidly operating commutator the current for a certa'in'interval indicated by the distance between two adja-' cent vertical lines is permitted to flow to the line with the line terminals connected in a certain manner to the signaling circuit through the commutator, whereas for the succeeding interval the connection of the line terminalsis reversed by the commutator, and

so on, Asa result the character of the current impressed on the line is somewhat as indicated by .the curve 2. This curve is.

drawn by reproducing portions of the curve of Fig. 1 included between adjacent vertical lines, reversing every alternate portion. It will be observed that the curve 2 has sharp angular portions at the point I where the reversal occurs. As is well known,

these sharp angles represent, so far as the effect of the current upon a selective circuit is concerned, a number of components having frequencies much higher than the frequency of alternation. between the commutator and the line, arranged to suppress currents having frequencies higher than a frequency exceeding the frequency of alternation by the frequency If a filter is placed sion properties, the ran e of frequencies transmitted maybe confined to a definite band as is common practice in multiplex carrier current systems.

. The curve 4 of Fig. 2 may be assumed to represent a portion of an alternating current signal wave and if this wave is commu tated at a rate indicated by the closeness of the vertical light lines, a'wave of the form indicated by curve 5 will be obtained. This .curve has propertiessimilar to the curve representing the output current of a carrier suppression modulator such as is common-in the art. In other words, the wave produced by commutating an alternating current represents a pure modulated wave containing frequency components of both side bands but havingno component of the carrier frequency itself. By passing the commutated current through a suitable filter, a smooth current such asindicated by curve 6 is obtained. If current of the'character represented by curve 5 or curve 6 is passed through a rectifying detector, a current having a frequency double that of the original si alin current represented by curve 4 will be 0 tained. In order to receive the signal from a wave of the type shown by curve 5 or curve 6 by rectifying the wave with a crystal or tube detector, it is necessary to supply at the receiver a wave of the frequencyof alternation, that 1s, a wave of the original carrier frequency. However, a reversing commutator running insyn'chronism with the modulating commutator will restore a wave such as shown by curve 5 or 6 to substantially the form of the signal wave indicated by-curve 4;

Referring ,to Fig. 3, a system is shown having telephone stations indicated at I and II. Each station may comprise the usual local battery telephone set including the microphone M, receiver R and may also be provided with a ringer S and a'hand generator G. The stations I and II may be-connected by local lines- L and .L to a toll line ML. The toll line or main line ML termi nates at each end in frequency converting apparatus comprising the commutators 10 and 12, respectively, and filters, the purpose of.

which will be described.

The commutators 10 and 12 are duplicates .of each other and may have any convenient or suitable design and are assumed to be driven in synchronlsm and inv phase with each other. No means for driving the commutators or for securing synchronism between them has been shown since arrangements for controlling distantly situated commutators in this manner are well known in the art. Furthermore, individual linessuch as the line ML may not be concerned in transmission of the synchronizing or control orjdriving currents since it is sufficient. to

have atcach terminal station a single syn;

,natin 1,'uss,41o

chronously driven shaft from which a large number of commutators employed, it may be, by a. corresponding number of lines terminating at that station are driven. In such case the control currents for the commutators may be transmitted over a separate circuit or may be superposed on one of the lines as is known in the art.

In 0 eration assuming that communicating re ation exists between the stations I and II, and that both receivers are removed from their switchhooks, voice currents origiin the station I pass through the filter F to one pair of brushes on the commutator 10. The filter LI!" is designed to pass freely essential voice frequencies but to suppress transmission of frequencies that are very much higher than those required for 3 ch transmission. In the system being ascribed, the commutators 10 and 12 are preferably driven at such a rate and the number of segments on each is so arranged that the frequency of alternation is substantially the same as or slightly above the highest frequency component of the essential speech requency range. The resulting commutator wave has therefore a frequency of alternation of say 2000 to 2500 cycles per second and is of the general type indicated by curve 5 rather than of the type indicated by curve 2, since the voicewave applied to the commutator is an alternating current wave of the general form indicated by the curve 4. The filter F in this instance may be a duplicate of the filter LF and is arranged to transmit freely currents of all frequencies up to and including the highest essential voice frequency, but to suppress currents of frequencies higher than th1s limiting frequency. This and the other filters indicated may each be designed in accordance with the principles. set forth in the United States patent to G. A. Campbell, 1,227,113 granted may 22, 1917.

It' was explained in connection with curves 5 and 6 that these curves represent a wave having frequencies comprising the upper and lower side bands without the carrier frequency component.- Since the carrier frequency in the present instance is of the same order as the highest essential voice frequency, these side bands produced by commutator 10 comprise two ranges "of frethe line ML. These inverted voice fro quencies are unintelligible to a high degree when converted directlyinto sound waves.

Due to the attenuating characteristics of the filter F, the commutated current will be smoothed out somewhat as indicated by the curve 6, although the curve is not intended as an exact representation of the wave transmitted through this filter since this filter transmits one side band only, while curve 6 more nearly represents a wave containing both side bands.

Turnin now to the opposite terminal of the line the filter F may be a duplicate of the filter F and the inverted voice freuencies received over the line pass through 31c filter F to a air of brushes on'the commutator 12. This commutator produces a reverse effect to the modulating action performed by commutator 10, and its action ma be understood from referring to Fig. 2 an considering that the commutator 12 inverts every alternate current fragment represented by the curve 5 or 6, the effect of which is to produce a wave of the general form of curve 4. In this inverting action, due largely to interruptions at the edges of the commutator segments; a number of high frequency components will be produced and it is the function of the filter Ll? which is a duplicate of the filter LF to suppress the transmission of currents having frequencies abo've the voice range. The restored voice frequencies pass therefore over the line L" and are received at substation II.

Transmission from the station II to station I is accomplished in precisel the same manner as has been described 0 transmission in the opposite direction. Voice currents received from the line L and the filter LF are commutated at 12 producing side bands of frequencies respectively in and above the essential voice frequency range. The lower of the two side bands is transmitted. by the filter F, the line NEL, filter F to the commutator 10 where it is commutated-to produce speech current which is transmitted bythe filter LF and the line L to the station I. q

A system of the character above described possesses a high degree of secrecy and the use of a commutator for modulating purposes permits the same degree of secrecy to be obtained by the comparatively simple circuit arrangements that have been de scribed so far that could he obtained only by considerably more complicated apparatus if a modulator of the vacuum tube type for "example were employed.-- 'The reason for this is that the commutator 10 produces the two side bands without at the same time transmittin double frequencies or unmodi fied voicerequencies or unmodulated carriercomponents',so rne orall of which would be transmitted by a vacuum'tube modulator and would need to be eliminated by additional circuit elements. The currents ac' tually transmitted over the line ML are not understandable if received by an ordinary telephone receiver or if rectified by the usual rectifying detector, and can onl be made to produce understandable space by being commutated or by being received in a rectifying detector to which a wave of the car- 'rier frequency is simultaneously applied. Ringing currents are transmitted in a manner entirely similar to that described of the voice currents from either generator G to actuate the ringers S. a

In Fig. 4, the line ML is shown terminating at each end in two sets of apparatus for enabling simultaneous transmission over the line of two carrier telephone conversations between the pairs of stations III, IV and V, VI, and, in addition, connections for a third' telephone conversation employing currents of voice frequency transmitted between circuits 15 and 16. The line ML is shown connected 'at one end to the band filters F and F and to the low pass filter F The opposite terminal has band filters F and F, respectively duplicates of F and F and a low pass filter F The low pass filters F, and F may be duplicates of the filters F and F of Fig. 3, and are shown situated between the main line liIL and the terminal circuits 15 and 16 respectively which ma 'be assumed to lead to low frequency signaling sets, such as ordinary telephone stations, and also if desired to composited telegraph apparatus. The commutators 17 and 18 may be driven from the same shaft and also-the commutators 19 and 20 may be driven from one shaft, but either the number of segments or the driving arrangements are such that the commutators 17 and 19 produce a less number of alternations per second than do the commutators 18 and 20, it being understood that the commutator-s 17 and 19 run in synchronism with each other, as do also the commuta-tors 18 and 20 with each other. The filters F andF are arranged to transmit a band of frequencies includlng the frequency components contained in the modulated wave from commutator 17 or 19. The filters Fiand F are arranged to transmit a band of fnequencies different from that transmitted by the filtersF and F and including the frequencies involved in the trans mission through the channel containing com- F and F may be arranged to pass frequencies from about 4000 to 6000 cycles, while the filters F and F may be arranged to transmit frequencies from 7000 to 9000. The commutators l7 and 19 may then be run .at such a rate -as to have a frequency of alternation of 3800 and the commutators 18 mutators 18 and 20. For example, the filters id gojh i; h vea rate of alternation of 5800. Wit this sort of. arrangement, it willhe seen that the upper side band 1's transinitted ineach case, it being assumed that this' side band comprises a range of frequen- L Efl-F" and F 'qlfaredesignedto pass currents of 'thdSsntiaLlvoice frequencies and to suppress those of higher frequencies The operation of the system of Fig. 41s as follows Assuming that-the commutators and the filters arearranged to employ frequen'cies'iabove indicate the switches 21 will be thrown to engage'the upper contact and voice currents originating in station III, for example, pass through the transformer T and the filter F", switch 21 to a pair of brushes of the commutator 17. The resulting upper side band of the frequencies produced by the commutator 17 are transmltted through the band filter F, but are prevented fromtransmission'through any of theother filters associated with the system except the filter E which is selective of these frequencies. 'These frequencies are transmitted to thecominutator 19 and from there through 3 the filter F to the telephone set of the sta- -tion IV. Signals are transmitted from the station IV to the station III in an entirely similar manner, the commutator 19 serving to produce a modulated wave of which the upper side band is selectively transmitted by filters F and F The commutator 17 reproduces the voice frequencies, which are transmitted by the filter- FT, while thehigher components are suppressed. Voice currents originating in the telephone set of the station V are transmitted through the transformer T and filter F to the commutator 18 which produces a voice modulated wave, the upper side band of'which is transmitted by the y band filter F line ML and vband filter F to the rectifying commutator 20. The received. voice currents pass through the-filter F to the telephone set of the station VI.

Transmission in the opposite direction is accomplished in a similar manner. a

If the switches-21 are thrown so as to include the battery 22' in circuit with the low frequency brushes of the commutators, and if the voltage of this battery exceeds the voltage amplitude of the impressed'voice waves, it is evident that the wave applied to the commutators will be of the general form indicated by curve 1, in which case the modulated wave produced by the commutator will comprise not only the two side bands, but a certain amount of unmodulated carrier component. If it is desired to transmit the unmodulated carrier component, the

filtersFand l3, ]i and F must, of'course, be designed to have a transmission range intransmission of the signal.

cluding the frequenc of-the unmodulated I carrier component. lso it is not necessary that the filters F, F, etc. should suppress 'one side band.- but if desired they may be arranged to transmit all of the components of the modulated wave having to do with the- The system arranged as in Fig. 4 is capable of transmitting two carrier telephone conversations without mutual interference, and in addition an ordinary low frequency telephone conversation originating in either of the terminal circuits 15 or 16, and destined for the opposite of these two circuits. By duplication of the apparatus shown other channels may, of course, be provided to any convenient and desired number, it being understood that the eommutators of each channel have a rate of alternation different from that of the commutators of any other channel.

If'it is desired to have the voice-range channel of the system of Fig. 4 a secret channel, the filters F and .F, may corre spond respectively to the filters F and F of Fig. 3, the circuit 15 leading to the commutator 10 and the circuit 16 leading to the commutator 12. These commute-tors may be -driven from the same respective shafts as the othercommutators, the number of segments and the driving arrangements being such as to give the frequency.

It has been described how both speech and ringing currents can be transmitted and received by converting them first intoalternating current waves and then back into signaling currents by means of commutators. The inventionis not, of course, limited to the transmission of speech and ringing signals in this manner,'but the means and method of the invention may be employed in various manners, and for transmitting other kinds of signals, as may be desired.

' What is claimed is:

1. The method of carrier telephony comprising commutating voice currents at a frequency high in comparison with the highest essential speech frequencies to produce a voice-modulated carrier wave and filtering the wave to confine the transmitted frequencies within substantially the extreme frequencies of the side bands.

I 2. The method of carrier telephony comprising commutating voice currents at a frequency high in comparison with the highest essential speech frequencies and selectively transmitting a fportiononly of the commutated currents. v

3; The method of carrier signaling which comprises separately commutating difierent required carrier message currents at difierentfrequencies all of which are high -,in comparison. with. the highest frequency component of said mes- Safii VOW??? P F l bands from quency components from transmission.

5. The method of Si aling which comprisesproducing signa ing currents, comt mutatin said currents at a fre uency at least as igh as the highest signal requency necessary for intelligible reception of the signals and filtering the commutated currents to restrict the transmitted range to frequency components lower than the frequency of commutation.

6. The method of producing one side band of a voice-modulated carrier wave which comprises commutating a voice current at the carrier frequency and suppressing the transmission of all resulting components except those comprised in one side band.

7. The method of transmitting speechwhich comprises commutating a voice current at a sustained frequency, transmitting one side band of the resulting frequency components and suppressing transmission of the other components. 8. The method of multiplex carrier wave transmission which comprises commutating each of a plurality of message currents at respectively different frequencies to produce side bands, suppressing transmission of one of the resulting side bandsproduced from each message current, and transmitting the remaining side bands simultaneously over the same path.

9. The-method of multiplex carrier wave transmission which comprises commutating each of a plurality of message currents at respectivel different frequencies to produce side ban s, suppressing one of the resulting side bands produced from each message current, transmitting the remaining side bands simultaneously over the same path and separating the frequencies representing the different messages by filtering.

10. The method of secret telephony which comprisescommutating speech current at a frequenc in the nei hborhood of 2000 to 2500 cyc es er .secon selectin for transmission the ower side hand of t e resulting frequencies andssuppressing the other fre- 11L In a slgnaling system, a source of signaling waves comprising a range of frequ ey components, a commutator for conve 115g sald waves into waves ofhigher frequency a line for'transmittin said waves of higher frequency, and a ter between sald commutator and line for restricting the frequencies transmitted to a range of frequencies representing the signaling frequen c1es and having substantially the same ex-' tent as the range embraced by said-signaling waves.

12. circuit. for transmitting signaling waves including two wave filters, at least one of which transmits electrical waves of the signaling frequenc and a commutator connected between sai filters'havin a frequency of commutation at least as igh as e upper essential signaling frequencies. 13. A circuit including twowave filters one of which transmits frequencies of the essential voice range, and a commutator connected between said filters having a freuency of commutation at least as high as t e upper essential speech frequencies.

14. In combination, a commutator, circuits for conveying currents to and from said commutator, a voice-frequency filter in one clrcuit, and a filter for frequencies above the voice range in the other circuit.

15. A two-Way translating circuit comprising a transmission line, a" transmit-receive terminalcircuit having means for transmitting current to and receiving current from said line, a current-reversing commutator connected in circuit between said terminal 1 circuit'and said line, and a filter in circuit between said commutator and said line.

16. Ina signaling system, a source of alternating current signal waves, a commutator for producing from the energy of said waves a signal modulated wave for transmission to a distance, whereby the unmodulated carrier wave component of said signal modulated wave is suppressed, and a band filter for suppressinga portion of the modulated current components before transmission to a distance.

17 In a signaling system, a source of signaling waves, a commutator for converting the energy of said waves into a signal-modulated wave, and a filter for suppressing one side hand of frequencies of said modulated.

wave and transmitting selectively the remaining side band.

' 18. In a signaling s stem, a signal transmission line, a plura ity of signal-current circuits associated therewith, and commuta-' tors connected one in circuit between each such signal-current. circuit and said line, said commutators being arranged to have respectively different rates of commutation.

19. In a signaling system, a transmission line, a lurality of signal-current circuits associate therewith, commutators connected one in circuit between each such signaLcurrent circuit and said line, said commutators arranged to have rates of alternation and a wave filter con.-'

' line.

nected between each commutator-(and 20. A signaling 'tiplex main transmlssion line cooperating at I each end with a plurality of signaling current branches, each of said branches being connected to said main line through a separate commutator, the said commutators 'at one terminal of said main line having respectively difierent "ratesof commutation.

21. In a carrier transmission system including a main transmission line having stations, a carrier wave channel com rising a transmitting signaling set at one 0 said stations on said line and a receiving signaling set at another of said stations on said line, and means comprising a commutator and a wave filter for connecting each of said sets with said line.

22. A multiplex signaling station for a line, comprising a uniformly rotating shaft, a plurality of rota'ting commutators driven from said shaft and arranged .to-have respectively difl'erent current commutating rates, each of said commutators being provided with sets of terminals for respective associated circuits, a different signal circuit connected to one set of terminals of each of said commutators, a plurality of filters connected in common to said line, the opposite sets of terminals of each of said commutators being connected to one of said filters.

23. In a signaling system, a circuit carry ing signaling currents comprising a range of frequency components, a line, a commutator connected between said circuit and said line respectively difierent for; converting said signaling currents into the;

system comprising a mulalternatingcurrents for, transmission over 40 said-(line, .saidcommutator having a frequencyfof commutation high in comparison with'fthe-upper essential signaling frequen- 'cies,and a filter connected ,between the commutator and the line for restricting the frequenciestransmitted to the line to a range substantiall braced by t currents .and lying substantially wholly to one side of the frequency of commutation.

equal in extent to that em-' e components of said signaling 24. In a signaling system, a circuit carrying signaling currents, a line, a commutator connected betw'eensaid circuit and'said line for converting said signaling currents into alternating currents for transmission over said line, said commutator having -a frequency ofcommutation high in comparison with the highest essential frequency component of said signaling currents, and a filter connected between the commutator and the line for suppressing the transmission of frequencies higher than the frequency ofcommutation and for selectively transmitting lower frequencies. Y I

25. A- two-way terminal translating circuit for a carrier channel comprising a com mutatonmeans including said commutator for producing a modulated carrier wave for transmission and means including said commutator for deriving signals from received carrier waves.

In witness whereof, I hereunto subscribe my name this 17th day of December, A. D.

RALPH V. L. HARTLEY. 

